CRUSTAL-SCALE, REACTIVATED BOUNDARY BETWEEN WALKER LANE AND CENTRAL NEVADA SEISMIC BELT REVEALED BY GEOLOGIC MAPPING

The Sand Springs Range and southern Stillwater Range, in the western Great Basin, expose a long-lived boundary between the Walker Lane (WL) and central Nevada seismic belt (CNSB) that involves Mesozoic metamorphic tectonites, diverse Jurassic(?), Cretaceous, and Tertiary intrusions, and Oligocene – Miocene extrusive igneous rocks. Recent Basin and Range normal and oblique-slip faults as well as Walker Lane strike-slip faults cross-cut these rocks. The 1954 Fourmile Flat quake several km S produced right-lateral slip (Doser, 1986).

2019 undergraduate research and 2022 Angelo State University Field Camp 1:8000-scale mapping focused on the SW Stillwater Range. The most detailed published geologic map of this area is 1:250,000 scale (Willden and Speed, 1974). Techniques applied included: a) distinguishing and measuring flow foliations, compaction foliations, original sedimentary bedding, metamorphic foliations, and lineations, b) working out Mesozoic - Cenozoic stratigraphy, c) distinguishing igneous map units by phenocryst compositions and percentages, d) constructing a grid of cross-sections, and e) geochemical analysis of Cenozoic igneous rocks. Eighteen map units were distinguished: 4 Mesozoic meta-sedimentary units, 4 Tertiary ash flow tuff and lava units, including the capping 13 Ma Bunejug basalt, 7 Mesozoic and/or Tertiary intrusive units, and 3 Quaternary units. The meta-sedimentary succession of black meta-shale, quartz arenite, and meta-limestone correlates to similar late Triassic – Jurassic interval in the southernmost Clan Alpine Mountains. High-angle faults strike NW, NE, and E-W. Small Cretaceous and/or Tertiary andesite, gabbro, and diorite plutons intruded near and along mapped faults. Major element geochemical analyses show two tuff samples are compositionally similar to Job Canyon and Poco Canyon tuffs.

A long-lived, reactivated, and deep WL-CNSB boundary fault interpretation is supported by: a) diverse phaneritic plutons within fault zone indicate preferential intrusion along faults and rapid uplift after intrusion, b) unusual ENE, E-W fault orientations on faults cross-cutting Tertiary units, c) NW- and NE-striking faults crosscut Quaternary units. The E-W-striking gold-silver bearing Summit King fault is likely a reactivated Luning-Fencemaker thrust fault ramp.